This invention relates to forming cementitious slurries from drilling fluids and to using such slurries in drilling and cementing.
Cementing of the annulus between an oil well casing and the surrounding borehole has been practiced for over 90 years. Long ago, it was recognized that it would be advantageous to solidify drilling fluid in the annulus so as to reduce the cost of the cementing of the casings. Over the decades, various attempts have been made along this line as evidenced by the disclosures in Williams et al., U.S. Pat. No. 2,649,160 (Aug. 18, 1953), Tragesser, U.S. Pat. No. 3,557,876 (Jan. 26, 1971), and Miller et al., U.S. Pat. No. 3,887,009 (Jun. 3, 1975). However, such techniques, while presumably operable, have failed to achieve commercialization even after the passage of many years. Another attempt in this direction is described in Wyant et al., U.S. Pat. No. 3,499,491 (Mar. 10, 1970), wherein Portland cement is used in an aqueous drilling fluid to produce a mud concrete. Portland cement, however, is very sensitive to the water/solids ratio. Even under ideal conditions, only small increases in the amount of solids results in a very viscous mass. On the other hand, and only a slight decrease in the amount of solids results in a composition which sets up to give a very weak structure. These problems are amplified when attempting to use Portland cement in a drilling mud. Thus, while this technique has been used commercially, it has serious drawbacks.
Expanded use of extended reach drilling and slim hole drilling has increased the problem of removing an incompatible drilling fluid before cementing, because the more narrow annulus created in these techniques create makes conventional cementing far more difficult.
Generally, drilling fluids are tailored to the characteristics of the formation into which the borehole is being drilled so far as density and rheological properties such as viscosity are concerned. Thus, even if the composition is tolerant of substantial variations in solids to liquid ratio so far as strength and viscosity are concerned, the density is still changed by the addition of a cementitious material to the composition.
Finally, when the drilling operation is complete there remains the problem of disposing of the used drilling fluid.